1. Field of the Invention
The present invention concerns an improved isolation member for use in a non-electric detonator cap and an improved detonator cap including the same. More particularly, the present invention concerns an isolation member that provides alternate paths by which a signal emitted by a signal transmission line can reach a pyrotechnic or explosive charge in the detonator cap.
2. Related Art
The use of isolation members in non-electric detonator caps which are to be assembled to fuses of a type capable of transmitting a static electric charge is known in the art, as shown by U.S. Pat. No. 3,981,240 issued Sep. 21, 1976 to E. L. Gladden. That Patent discloses the use of signal transmission lines, i.e., fuses, of the type disclosed in U.S. Pat. No. 3,590,739 issued Jul. 6, 1971 to P. A. Persson. Such fuses, commonly referred to as "shock tubes", comprise an elongated hollow tube made of one or more synthetic organic polymeric material(s) (plastics) containing on the interior wall thereof a coating of reactive material such as a pulverulent high explosive and reducing agent, for example, PETN or HMX and aluminum powder. The coating of reactive material on the interior wall is quite thin and leaves the tube hollow, providing an open channel or bore extending the length of the tube. When the reactive material is ignited, as by a spark igniter or a detonator cap used as a signal transmitter, or any other suitable means, ignition of the reactive material propagates an initiation signal through the bore of the tube. If the tube is properly connected to a receptor detonator cap, the initiation signal will initiate detonation of the cap. (As used herein, the "receptor" detonator cap is the cap which is to be detonated by the initiation signal transmitted through the tube or other signal transmission line.)
Other patents concerning such shock tubes and the manufacture thereof include U.S. Pat. No. 4,328,753, issued May 11, 1982 to L. Kristensen et al and U.S. Pat. No. 4,607,573 issued Aug. 26, 1986 to G. R. Thureson et al. As disclosed in the Thureson et al Patent, the reactive material may comprise a thin coating or dusting of a mixture of an explosive such as PETN, RDX, HMX or the like, and a fine aluminum powder, and the shock tube may be a plural-layer tube. For example, as disclosed in the Kristensen et al Patent, the inner tube may comprise a plastic, such as a SURLYN.TM. ionomer, to which the reactive powder will adhere and the outer tube may be made of a mechanically tougher material such as low or medium density polyethylene. (SURLYN is a trademark of E. I. Du Pont de Nemours & Co. for its ionomer resins.)
U.S. Pat. No. 4,757,764 issued Jul. 19, 1988 to G. R. Thureson et al discloses signal transmission lines comprising tubes as described above except that instead of an explosive powder of high brisance (e.g., PETN or HMX), the reactive material is a low velocity deflagrating material. Use of a deflagrating material provides a reduced speed of transmission of the initiation signal propagated through the tube as compared to shock tubes. Such deflagrating material tubes are referred to as low velocity signal transmission lines ("LVST.RTM. lines"). Numerous deflagrating materials are disclosed in U.S. Pat. No. 4,757,764, including manganese/potassium perchlorate, silicon/red lead, and zirconium/ferric oxide, to name but a few of the many disclosed in that Patent starting at column 3, line 48. As pointed out at column 4, line 47 et seq. of that Patent, LVST.RTM. lines transmit an initiation signal by means of a "pressure/flame front" principle whereas shock tubes, when ignited, produce a "shock wave initiation signal" which travels through the tube. Both types of tubes, shock tubes and LVST.RTM.lines, as well as detonating cords, especially low-energy detonating cords, may be used to initiate detonator caps for use in demolition, mining and other systems. Such tubes and cords are collectively referred to herein and in the claims as "signal transmission lines".
Signal transmission lines of the type comprising a tube containing a metallic powder such as aluminum as part of the reactive material are capable of transmitting a static electric charge which may result in premature detonation of the receptor detonator cap, which can of course have catastrophic results. Accordingly, the invention of the above-mentioned Gladden U.S. Pat. No. 3,981,240 provides a fuse-retaining bushing (28) made of a semi-conductive plastic material. The bushing provides a "stand-off", i.e., a space, between the signal-emitting end of the initiating fuse (26) and the target of the initiation signal which, as illustrated in the Gladden Patent, is a primer or booster charge (20). The bushing isolates the signal-emitting end of the signal transmission line from the target by a thin, flat rupturable membrane (40). The bushing further provides a shunt path for transmitting static electric charges from the signal-emitting end of the initiator fuse to the metallic shell or casing (12) of the detonator cap, thereby bleeding off static charges before they reach a potential high enough to cause a spark which could penetrate the membrane and ignite the cap charge to prematurely detonate the cap.